As of right now sparrow go as far as rare but not legendary or exotic; bungie needs to make more. Also while there at it they can give the ability to jump to the legendary once and the exotics can come with movement abilities like glide, blink, lift, and double jump
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There are currently legendary sparrows in the game. You can get one from the raid. Also you can buy a rare sparrow and purchase an upgrade from Xur to make it legendary.